Significant growth in both high-frequency wired and wireless markets has introduced new opportunities where compound semiconductors such as SiGe have unique advantages over bulk complementary metal oxide semiconductor (CMOS) technology. With the rapid advancement of epitaxial-layer pseudomorphic SiGe deposition processes, epitaxial-base SiGe heterojunction bipolar transistors have been integrated with mainstream advanced CMOS development for wide market acceptance, providing the advantages of SiGe technology for analog and RF circuitry while maintaining the full utilization of the advanced CMOS technology base for digital logic circuitry.
SiGe heterojunction bipolar transistor devices are replacing silicon bipolar junction devices as the primary element in all analog applications. A typical prior art SiGe heterojunction bipolar transistor is shown in FIG. 1. Specifically, the prior art heterojunction bipolar transistor includes an n+ subcollector layer 10 having a layer of n− Si collector (i.e., lightly doped) region 12 formed thereon. The transistor further includes p+ SiGe base region 14 formed on the lightly doped Si collector region. One portion of base region 14 includes n+ Si emitter region 16 and other portions include base electrodes 18 which are separated from the emitter region by spaces 20. On top of emitter region 16 is an emitter electrode 22.
A major problem with bipolar SiGe transistors of the type illustrated in FIG. 1 is the presence of dislocations between the collector and emitter regions. When these dislocations extend between the collector region and the emitter region, bipolar pipe, e g., CE, shorts occur; Pipe shorts are a major yield detractor in SiGe bipolar technology.
In the prior art, it is known to incorporate carbon into a bipolar structure so as to form a carbon layer over the base in the SiGe region only. Such a structure is shown in FIG. 2 wherein reference numeral 24 denotes the grown carbon layer. This prior art technique which forms a C layer over the base in the SiGe region results in a narrow base width by hindering diffusion of the intrinsic base region. This result is shown, for example, in FIG. 3.
Carbon incorporation is typically employed in the prior art to prevent the out-diffusion of boron into the base region. For example, it is known that the transient enhanced diffusion of boron is strongly suppressed in a carbon-rich silicon layer, See H. J. Osten, et al., “Carbon Doped SiGe Heterjunction Bipolar Transistors for High Frequency Applications”, IEEEBTCM 7.1, 109. Boron diffusion in silicon occurs via an interstitial mechanism and is proportional to the concentration of silicon self-interstitials. Diffusion of carbon out of the carbon-rich regions causes an undersaturation of silicon self-interstitials. As a result, the diffusion of boron in these regions is suppressed. Despite being able to suppress the diffusion of boron, this prior art method which forms C over the base in the SiGe region only is not effective in reducing pipe shorts.
In view of the bipolar pipe shorts problem mentioned above, there is a continued need for the development of a new and improved method for fabricating SiGe bipolar transistors in which dislocations between the emitter and collector regions are substantially eliminated, without narrowing the base width as is the case with prior art methods.